Modeling Hydrogen-Induced Fracture and Crack Propagation in High Strength Steels

We investigate hydrogen-induced crack initiation and subcritical crack propagation in high strength steels by addressing hydrogen transport and interaction with material deformation. Our experiments demonstrate that hydrogen reduces the fracture resistance of single-notched bend specimens of a lath martensitic steel while changing the morphology of the fracture surfaces from microvoid coalescence in the absence of hydrogen to a mixture of "quasi-cleavage" and intergranular. We modeled the onset of fracture in this steel by assuming that failure occurs by dislocation pile-ups against high angle grain boundaries whose strength is assumed to be reduced by hydrogen. The model reproduces the experimentally measured reduction of the fracture strength of the bend specimens as a function of the hydrogen charging pressure. We also present a model for subcritical crack propagation and arrest under sustained load in a hydrogen gaseous environment. The numerical results for a model high strength steel show that the velocity vs. stress intensity factor (V-K) curve exhibits the typical stages I and II. Interestingly, this model suggests that the existence of the two stages is explained solely by stress-driven diffusion of hydrogen.